Advanced age is a major risk factor for cardiovascular disease, which remains the leading cause of death in the aged population. Aging is associated with impairments in ischemia-induced neovascularization, which occurs via two pathways: angiogenesis (sprouting of existing vessels) and vasculogenesis (de novo vessel formation by circulating endothelial progenitor cells, or EPCs). Our long term goal is to understand the mechanisms of vasculogenesis, thereby providing rational therapeutic strategies for augmenting or preventing neovascularization in aged or disease states. We have recently demonstrated that stromal cell-derived factor-1 (SDF-I) is selectively expressed at sites of ischemial injury via the transcription factor HIF-1, and is a critical determinant of EPC trafficking and recruitment during vasculogenesis. In addition, recent studies have demonstrated that HlF-1 activity is markedly diminished during the aging process. Thus, it is our hypothesis that trafficking of vascular progenitorlstem cells to sites of injury is impaired during aging because of a failure to upregulate HIF-induced SDF-1 expression in response to an ischemic stimulus. We believe that these acquired deficits in stem cell trafficking rather than stem cell depletion are responsible for the normal vascular changes that occur during aging and may account for the increased incidence of vascular disease in the aged population. In this proposal, Specific Aim I will establish the physiologic role of HIF- induced SDF-1 expression in mediating normal progenitor trafficking using a gene targeting approach. In Specific Aim 2, we will determine how this normal trafficking mechanism is affected by aging, specifically examining the contributions of both the environment (soil) and progenitor cell function (seed) in an animal model. In Specific Aim 3, we will perform reliable assays of cellular function in human subjects examining both soil and seed to determine the relative contributions/impairments of each to stem cell trafficking in the aged. This will define normative data for stem cell trafficking in healthy aging and correlate possible deficiencies in these functions with age-related vascular disease. Finally, in Specific Aim 4 we will investigate the use of genetically modified endothelial progenitor cells as ischemia- targeted agents to restore SDF-1 expression and normalize EPC trafficking, thereby augmenting blood vessel growth and restoring neovascularization in aged subjects. We believe that understanding age related defects in stem cell trafficking will be important for the design of rational therapeutics to prevent and treat vascular aging.